Originally posted by reh321 I was responding to comments about sensor size.
My post was about sensor size as well, arguing that sensor size is not the dominant factor for the size of a lens.
Even for a very small sensor, you can decide to collect a lot of light (-> large lens) or not so much light (-> smaller lens). As you agreed, the f/1.9 lens for the Q collects only ~1/20 of the light of an FF f/1.9 lens and that's the reason why it's front element can be so much smaller.
Originally posted by reh321 My light meter asks for f-stop, plus ASA, to determine shutter speed; even when this light meter was constructed, exposure depended on the three "triangle" components, not on whether I was using medium format, 35mm, or even Minox film.
That's because your light meter aims for a specific "photons per mm^2" exposure. The latter doesn't depend on the sensor size.
The larger the sensor, though, the more photons need to be gathered by the lens in order to maintain the same "photons per mm^2" figure. That's why an FF f/1.9 lens needs to collect 20 times more light than a f/1.9 lens for the Q. Note, however, that the DOF and noise levels will be very different for images shot with the same exposure parameters, in other words, the FF lens at f/1.9 doesn't just serve a bigger sensor, it actually creates a different image when used with the same parameters (the ones your light meter tells you).
The above makes it tempting to attribute "shallow DOF" with larger sensor size, but in reality, it comes from using "faster" lenses (as f/1.9 in "FF currency" is much "faster" than f/1.9 in "Q currency").
In a nutshell, light meters are designed to create the same exposure across formats, not to create the same images across formats (they'll differ in DOF and noise).
Again, I don't think this is the right thread to discuss such matters with the necessary detail in explanation but I just wanted to clarify why my earlier statements stand.